I won't have the definitive answer for your questions, but I think I can clear up a few things. A lot of generalities.
There is a difference between the heartwood (dark in your photo) and the sapwood of the log, and then as it is processed into lumber. Differences in range of movement - generally sapwood is less stable, and durability - sapwood is more attractive to bugs and the smaller life forms that cause decay. Sapwood is avoided 100% in some species, and others will show hardly any visible difference and is included in the use of the board. As lumber becomes more expensive and hard to get, more sapwood is allowed. Sapwood is sometimes hard to see in White Oak, and nearly impossible to determine in Poplar. It is stark white in Padauk, contrasting strongly with the orange-red of the heartwood, so is avoided or used as a decorative element.
Across the width of a board, you can start at one edge with some sapwood/quartered grain, then quartered heartwood, transition to rift, then to flat sawn, then rift and back to quartered and sapwood. This is driven by the orientation of the growth rings to the face or edge of the board. In any one board, I think you will find the propensity for movement to be similar across its width at any two or more comparative points along the length. Microscopic differences (that do not matter to us), perhaps.
It is apparent that old growth - or high ring density - lumber has two characteristics that may enter this discussion. One, the more dense the rings, caused by slower growth, the more predictable the movement across the board. Not more or less movement, but predictable, since any one ring, or group of rings, will not cause the board to move in an unpredictable direction. With lower ring density (modern Poplar, for instance), an errant ring or three may cause the board to cup or move in an unexpected manner.
The second characteristic of old growth lumber is durability in exposure in service. High ring densities help a piece of wood last in the weather - is more rot resistant and more stable than another of the same species that is grown faster and with lower ring densities. This is why we see old growth Poplar (all heartwood) harvested from the original forest and made into cabin logs still sound after 200 years of exposure, where new Poplar lumber will sprout fungi after two seasons in exposure.
Generally, old growth trees are slow growing trees located in forests where competition for light and water is keen, so the rings are close and tight. Close ring density also yields wood that is more desirable for musical instruments since the density provides better tonal qualities than the lower ring density.
Variability in ring spacing is not a factor of the sunny side of the trunk, as I understand it, but more a factor of the tree's environment while it was growing and adding rings thru the seasons. Wide summerwood in the rings means a wet Spring and Summer - good growing conditions, just as narrow and close indicates less rain/water. This is why slices thru rings, or borings are taken, to determine local historical weather patterns. The rings have recorded it all. It is possible to see fire scars, bug invasions, storm damage and more by looking closely at the rings.
If a tree had another tree fall on it while it is growing, it may add wood thickness to one side of the log to shore itself up from the weight of its fallen neighbor. Many years later, if it is processed into lumber, the lumber in that part of the log may be reaction wood that moves as it is being sawn, long after it is made into boards and then kiln dried. Often this reaction wood can cause problems in the shop of finished products, since the reaction wood - uneven ring density - will move unpredictably.
As for the 'straws' causing or regulating movement, the difference in movement between a quartered board and a flat sawn board are seen by examining the end grain. A section of a log - a cookie - will want to move more along the rings (flatsawn) than from the center of the log to the sapwood (quartered).
I always assumed the reason this is so is due to the the latewood, or wood that is less porus, and the way it moves. This is where Dr Gene should come in.....
Also, if you don't have a copy of Bruce Hoadley's book "Understanding Wood" get a copy. Great nerdy reading, but lots of knowledge about our favorite material.